Instituição: University of WollongongDepartamento: Department of Civil, Mining and Environmental EngineeringAno: Janeiro/2012

Han - 2012.pdf

Abstract:
Ballast is one of the most significant components in rail track structure. It supports the rail and sleeper by transmitting the traffic load to the subgrade. Due to increasing traffic congestion and the price of fuel, the demand for high speed trains has been increasing on a daily basis. This contributes to a more permanent deformation and degradation of the ballast layer. Cyclic loads from heavy haul trains degrade and foul the ballast, directly contributing to track settlement. An understanding of how ballast reacts during cyclic loading plays a key role in reducing the maintenance costs of railway tracks, while optimising passenger comfort. Currently, numerous research studies have been carried out to understand the behaviour of ballast during cyclic loading. However, laboratory investigations and field assessments alone cannot provide a full insight into complex ballast breakage mechanisms and associated deformation when the discrete and heterogeneous nature of granular materials is considered. On the other hand, computer simulations using the Discrete Element Method (DEM) provides enough information from the particle scale level to help understand the deformation and breakage mechanisms that occur under complex cyclic loads. In this research DEM simulations using PFC2D were conducted to examine the degradation and deformation of ballast during cyclic loading, and a case study involving the Bulli track north of Wollongong city is also included. A cyclic biaxial simulation was conducted to supplement the DEM model for track behaviour at different frequencies during cyclic loading. The DEM results were similar to the laboratory results, which indicated that due to an irregular re-arrangement of the particles, most particle breakages and axial strains occurred during the initial cycles, and there was also some corner breakage at a low number of cycles. Particle breakage plays a key role in affecting the permanent deformation of ballast. An analysis of three distinct frequency regions showed that bond breakage and axial strains increased rapidly at , and then increased slightly at . Deformation and breakage increased significantly when . The DEM study indicated that the particles were first sliding and rolling to maintain a packing arrangement, albeit with some corner breakage, followed by particle bodies splitting as the internal forces increased. The half track simulation exploiting symmetry that was applied to the Bulli track was validated with field data under realistic train loading. It was demonstrated that particle breakage plays a key role in ballast behaviour under cyclic loading. Three specific measurement points (i.e., near the lateral boundary, the top layer under the edge of the sleeper, and the bottom layer under the edge of sleeper) were used in the model. The lateral strains and broken bonds increased significantly during the initial cycles, but a steady, static deformation and breakage was attained as the cycles increased. The lateral strain and particle breakage near the lateral boundary was much lower than under the edge of the sleeper. Particle breakage also controls the development of contact force (CF) chains in the direction of the major principal stress under cyclic loading. Those particles actually under the sleeper sustain more uniform CF chains and breakage compared to particles near the lateral boundary. Particle breakage and re-arrangement leads to more uniform CF chains in the direction of major principle stress, but as the cyclic loading continues, more particles are broken and rearranged, which causes the ballast to become denser and generate increased lateral displacement.